Hydraulic steering mechanism for marine drive

ABSTRACT

A locking valve is connected between the pressure fluid source and the orbitrol helm in a hydraulic steering system to confine the pressure fluid in the helm and the double acting steering cylinder in the event of failure of the pressure fluid source.

United States Patent 1 1 Lohse et 21.

[Ill 3,913,517

[451 Oet.2l, 1975 I541 HYDRAULIC STEERING MECHANISM FOR MARINE DRIVE Inventors: Lawrence L. Lohse, Fond du Lac;

William E. Hackbarth, St. Cloud. th of Wis.

[73] Assignee: Kiekhaefer Aeromarine, Fond du Lac. Wis.

[22] Filed: Mar. 29, 1974 [211 App1.No.:4S5.997

152} U.S. Cl. /35; 115/41 HT [51] Int. Cl." i. B6311 25/42 [58] Field ofSearch.-. 115/3534 R.41 R.41 HT. 115/12 A,18 RL114/144 R. 91/467;

[56] References Cited UNITED STATES PATENTS 2.918.135 12/1959 Wittren 180/792 R 3.059.717 10/1962 Muyer e1 :11 180/79,] R 3.272.085 9/1966 Hujmzi 91/420 3.374.627 3/1968 Hehel et ul. 60/567 3.508.400 4/1970 Mereier 114/150 3.534.703 10/1970 EkI1]111.... 115/35 3.596.626 8/1971 Budrus H 115/35 3.654.889 4/1972 Bergstedti 115/35 3.707.200 12/1972 Cumer 180/79: R

R25.126 2/1962 Churlstm.....,.. 91467 OTHER PUBLICATlONS Saginaw Steering Gear Division of General Motors Corp. Catalog No. 66. 1965. pp. 27-28.

Primary limminvr-Trygve M. 1:1li\

Arr/slum Iituminw-lesus D. Sulelu Attorney. Agent. m Firm Andrus. Seeules. Sturke & Suwull [57] ABSTRACT A locking valve is connected between the pressure fluid source and the urbitrol helm in a hydraulic steer" ing system to confine the pressure iluid in the helm and the double acting steering cylinder in the event 01' failure 01 the pressure fluid source.

4 Claims. 8 Drawing Figures US. Patent Oct. 21, 1975 Sheet 2 of7 3,913,517

. HNUEMNH Wm NMWM U.S. Patent 0m. 21, 1975 Sheet 3 of7 3,913,517

U.S. Patent 0:.21, 1975 Sheet4 of? 3,913,517

U.S. Patent 0a. 21, 1975 Sheet 5 of7 3,913,517

US. Patent Oct. 21, 1975 Sheet 6 of 7 3,913,517

US. Patent Oct. 21, 1975 Sheet 7 of7 3,913,517

HYDRAULIC STEERING MECHANISM FOR MARINE DRIVE BACKGROUND OF THE INVENTION This invention relates to an hydraulic steering mechanism for a marine drive.

Various forms of hydraulic steering mechanisms have been commonly employed to actuate a dirigible propulsion unit or a rudder unit to provide for steering control of a boat.

In such mechanism it is customary to operate power steering cylinders by an orbitrol helm which requires a source of fluid pressure. In the event the pressure source fails the orbitrol helm normally has a check valve bypass between the discharge outlet and the inlet to tend to retain the fluid in the helm for emergency manual operation without a pressure source input. Nevertheless this emergency operation is generally at the expense of some drainage of fluid from the system and the emergency steering becomes ineffective in a relatively short time.

SUMMARY OF THE INVENTION In carrying out the invention a locking valve is connected between the pressure fluid source and the orbitrol helm to confine the fluid in the helm and the power cylinder means or the separate cylinders in the event of failure of the pressure fluid source.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings illustrate the preferred embodiment of the invention as presently contemplated by the inventor.

In the drawings:

FIG. 1 is a vertical longitudinal section of the stern drive unit shown as attached to the transom of the boat;

FIG. 2 is a vertical transverse section of the drive unit taken on line 22 of FIG. I coincident of the vertical drive shaft axis;

FIG. 3 is a detail section taken on line 3-3 of FIG. 1 and showing the tilt trunnion and bellows construction;

FIG. 4 is a vertical section of the mounting plate taken on line 44 of FIG. 3 and showing one of exhaust passages;

FIG. 5 is a rear elevation of the boat assembly unit comprising the mounting plate and the tilt housing;

FIG. 6 is a section taken on line 66 of FIG. 1 and showing the steering and trim mechanism in top plan view with parts broken away and sectioned;

FIG. 7 is a diagram showing the hydraulic circuit for operating the steering mechanism; and

FIG. 8 is a diagram showing the hydraulic circuit for operating the trim cylinder.

DESCRIPTION OF THE PREFERRED EMBODIMENT The stern drive ofthe present invention, as illustrated in the accompanying drawings, comprises in general a boat mounting assembly I and a drive assembly 2.

The boat mounting assembly 1 comprises an outer mounting plate 3 adapted to be secured to the rear transom 4 of a boat and to be sealed thereagainst to close the opening 5 in the transom.

The engine 6 is mounted inboard of the boat with its transmission 7 carried directly by the outer mounting plate 3.

The boat mounting assembly I additionally com prises a tilt housing 8 which is carried by mounting plate 3 to pivot upon a horizontal transverse tilt axis 9 by means oftrunnions I0 disposed in bearings ll of the mounting plate.

A hydraulically operated trim cylinder 12 is pivotally secured to tilt housing 8 on a transverse axis I3 parallel to tilt axis 9.

The outer end of the piston rod I4 for piston 15 operative in cylinder 12 is pivotally secured to mounting plate 3 in a recess therein extending through transom opening 5 with the pivot 16 disposed inboard of the boat and parallel to tilt axis 9.

The tilt housing 8 carries a hydraulic steering mechanism 17 adapted to be connected to the dirigible part of the drive assembly 2 for steering the latter.

Various hydraulic steering mechanisms 17 may be employed, that shown and preferred. comprising a pair of cylinders 18 each having a plunger 19 and extending rearwardly on opposite sides of the tilt housing 8.

The plungers 19 are secured to the tilt housing 8 by suitable universal ball and socket connections 20 each having a retainer plate 21 therefor.

The mounting plate 3 has an opening 22 there through for receiving the horizontal drive shaft 23 extending rearwardly from transmission 7 with a double universal joint 24 therein and with the free rear end of the shaft splined as at 25 for connection to the drive assembly 2.

A bellows 26 surrounds the universal joint 24 with its outer end sealed to the tilt housing 8 and its inner and sealed to mounting plate 3.

A bellows type seal 27 surrounds shaft 23 and is disposed between mounting plate 3 and the housing of transmission 7 to protect the universal joint 24 from bilge water and compensate for changes in volume within bellows 26 upon tilt movement of housing 8.

The transmission 7 is carred by side posts 28 extending upwardly from the inwardly projecting portion 29 of mounting plate 3 which surrounds the recess containing pivot 16.

The exhaust from engine 6 is conducted by suitable conduit 30 to openings 31 in the mounting plate 3 on either side of the same beneath trunnions l0 and from which openings 31 of the exhaust is discharged rearwardly of the boat.

The mounting assembly 1 described above may be secured to the boat by the boat manufacturer, and thus shipped as an integral part of the boat.

For this purpose, a removable protective covering, not shown, may be secured over the rear face of the mounting assembly I for shipment purposes to protect the splined end 25 of shaft 23 and the rear ends of steering mechanism 17.

The drive assembly 2 may be shipped as a separate unit to protect the same from possible damage, and can be readily secured to the mounting assembly 1 by the dealer or other purchaser without requiring any adjustments.

The drive assembly 2 comprises the upper gear housing 32 and the dirigible unit or lower gear housing 33.

The upper gear housing 32 comprises a casting which encloses the upper bevel gear shaft 34 and which depends downwardly along the face of the tilt housing 8 with a vertical sleeve 35 for receiving the upper end of the vertical drive shaft 36 and an intermediate steering support tube 37.

The upper bevel gear shaft 34 is hollow and splined on the inside to receive the splined end 25 of drive shaft 23 to effect a driving connection thcrebetween and at the same time provide for the necessary extension when the tilt housing and drive assembly 2 tilt upwardly on tilt axis 9.

The shaft 34 is mounted in tapered roller bearings 38 at the opposite ends thereof with the construction providing for turning the shaft end to end.

A bevel gear 39 has a shank supporting shaft 34 in one ofthe bearings 38, and is splined upon the shaft for assembly purposes.

The bevel gear 39 meshes with a bevel gear 40 on the upper end of the vertical drive shaft 36 to drive the latter.

The direction of rotation of vertical drive shaft 36 can be changed by turning shaft 34 end to end whereby the beveled gear 39 will engage the opposite side of beveled gear 40, as may be desirable to cancel out the engine torque reaction on the vertical dirigible units when employing dual drives side by side on the same boat. For this purpose the dual drives will have their dirigible units connected by a cross link 41 to maintain steering synchronization.

A seal 42 is disposed between the outer end of shaft 34 and the housing 32 to prevent leakage of oil from the housing through the open tubular shaft during shipment and also during operation of the drive.

Likewise. a seal 43 is disposed between the inner end of shaft 34 and the bearing retainer cap 44 secured to housing 32. to prevent leakage of oil from the housing during shipment and also during operation of the drive.

A neoprene gasket 45 is disposed between upper gear housing 32 and tilt housing 8 to prevent water from entering the region of the universal joint 24 inside bellows 26.

The vertical drive shaft 36 is splined to bevel gear 40 and supported therein by lock nut 46 threaded onto the upper end of the shaft in a recess in the gear.

The bevel gear 40 is supported in housing 32 by the tapered roller bearing 47 at the upper end of sleeve 35.

The vertical drive shaft 36 is mounted within a sleeve 48 of the lower dirigible unit 33 by means of a set of straight roller bearings 49 at the lower end and the tapered roller bearings 50 near the center of the shaft.

The tapered bearings 47 and 50 are opposed to each other in thrust and the nut 46 determines the pre-load thereof. For this purpose a needle thrust bearing assembly S1 is disposed between a flange 52 on the steering support tube 37 and the lower end of sleeve 35.

A suitable belleville spring washer S3 is disposed beneath nut 46 to maintain the preload of bearings 47 and 50.

The vertical steering support tube 37 is rotatably disposed within sleeve 35 by means of upper and lower needle bearings 54.

The dirigible unit 33 has an upper face to which the steering housing 55 is secured by bolts 56, with steering support tube flange 52 clamped therebetween.

The lower end of dirigible unit 33 constitutes the lower gear case 57 and skeg 58.

The lower gear case 57 contains the propeller drive shaft 59 mounted in suitable tapered roller bearings 60 therein. Shaft 59 extends rearwardly from gear case 57 through the seals 61 and carries the propeller 62 for the drive.

To complete the propeller drive connection. bevel gear 63 on the lower end of vertical drive shaft 36 meshes with a bevel gear 64 fixed on propeller shaft 59.

The shafts 34, 36 and 59, and the bevel gears 39, 40, 63 and 64 along with corresponding bearings 38. 47 49. 50. and the bearings 51 and 54 all operate in a closed oil chamber within the upper gear housing 32 and the lower gear housing or clirigible unit 33, and which chamber is maintained filled with oil at all times to a level corresponding to the position of the axis of shaft 34. For this purpose. a seal 65 surrounds sleeve 35 between it and the steering housing 55 to prevent leakage of oil between the two housings.

The top of upper gear housing 32 has an access opening with a sealed removable cover 66 therefor generally co-axial with shaft 36 to provide for machining the inside of the housing and for assembly of gear 40 and nut 46 upon the shaft.

The bellows 26 is protected by suitable overlapping of telescoping portions 67 of tilt housings 8 and 68 of mounting plate 3 curved concentric to tilt axis 9.

The steering housing 55 has an abutment ring 69 on each side of the unit and through which the corresponding steering cylinders 18 extend. The two power cylinders 18 have the combined function of a single double acting power cylinder.

Each cylinder 18 has a spherically shaped ring 70 thereon adapted to abut rearwardly against and into a socket in abutment ring 69 of a generally complemental spherical segment shapev Hydraulic pressures are maintained in the cylinders 18 bearing against the ends of the corresponding plungers 19 therein to effect a balance of forces maintaining the dirigible unit 33 in any steering position of rotation on the vertical steering axis which in the illustration coincides with the axis of shaft 36.

As shown in FIG. 7, operation of the cylinders 18 is effected by a power steering assist system comprising a source 71 of pressure fluid, an orbitrol helm or valve unit 72 and suitable locking valves 73 and 74.

The pressure fluid source 71 comprises a sump 75, a gear pump 76 driven by engine 6 and having its inlet eonected to the sump, a pressure conduit 77 leading from the pump discharge outlet to locking valve 73, and a pressure relief valve 78 in conduit 77 and which returns fluid from the conduit to the sump when the system is fully pressurized.

The orbitrol unit 72 may be of the type marketed by the Fluid Power Division of Eaton Corporation under the trademark CharLynn" and illustrated generally in US, Pat. No. Re 25,126 granted Feb. 20, [962 and in US, Pat, No. Re 25.291 granted Dec. 4, i962.

Locking valve 73 receives fluid from conduit 77 and transmits it to orbitrol control unit 72 through passage 79 and which unit 72 then returns the fluid through passage 80 to valve 73 and from thence to the sump through return conduit 81.

The orbitrol unit 72 is connected to the locking valve 74 by two conduits 82 and 83, and locking valve 74 is in turn connected to the left hand steering cylinder 18 by conduit 84 and to the right hand steering cylinder 18 by conduit 85.

The locking valve 74 comprises a housing having a central longitudinal bore 86 therein with a plug 87 closing each end of the bore.

An axial passage 88 in the inner end of each plug 87 leads to a lateral passage 89 which is connected to the corresponding conduit 84 or 85.

A spring biased check valve 90 is disposed axially of each plug to effect closure of the corresponding axial passage 88 therein.

Conduits 82 and 83 connect with the opposite ends of the bore 86 and are isolated by a valve plunger 91 disposed centrally of the bore.

The plunger 91 carries a pilot pin 92 extending axially therefrom and loosely operative in the corresponding passage 88 to unseat the check valve 90 therein when the plunger moves in the corresponding direction in response to a pressure differential in conduits 82 and 83.

When the steering wheel 93 is turned to the right. it thus effects an increase in fluid pressure in line 82 which then actuates plunger 91 to the left in FIG. 7 and admits pressure fluid through the right hand passage 88 unseating the corresponding check valve 90 and ex panding the left hand or port cylinder 18 to turn unit 33 in a direction steering the boat to the right.

At the same time movement of plunger 91 to the left effects unseating of the left hand check valve 90 by the corresponding pilot pin 92 thereby allowing the right hand or starboard cylinder 18 to contract and the fluid therein to flow out through conduit 85 and the corre sponding lateral passage 89 and axial passage 88 to bore 86 and thence through conduit 83 to the orbitrol control unit 72.

Conversely rotation of steering wheel 93 to the left effects an increase in fluid pressure in line 83 which then actuates plunger 91 to the right in FIG. 7 and ad mits pressure fluid through the left hand passage 88 unseating the corresponding check valve 90 and expand ing the right hand or starboard cylinder 18 to turn unit 33 in a direction steering the boat to the left.

At the same time movement of plunger 91 to the right effects unseating of the right hand check valve 90 by corresponding pilot pin 92 thereby allowing the left hand or port cylinder 18 to contract and the fluid therein to flow out through conduit 84 and the corresponding lateral passage 89 and axial passage 88 to bore 86 and thence through conduit 82 back to the orbitrol control unit 72.

Orbitrol control unit 72 is directly operated by man ual turning of the steering wheel 93 disposed at a convenient place in the boat for the operator.

Whenever unit 72 is being actuated by steering wheel 93, the flow of fluid through unit 72 between passages 79 and 80 becomes modified to effect a pressure differential between conduits 82 and 83.

Whenever steering wheel 93 is stationary, the orbitrol unit 72 tends to block the flow of fluid therethrough between conduits 82 and 83. At the same time locking valve 74 is neutralized and locks the fluid in the corresponding cylinders 18 to thereby prevent turning of unit 33 by external forces upon the unit.

The locking valve 73 is identical in construction to locking valve 74 just described in detail. Consequently, so long as pump 76 is operating power fluid is tending to flow through line 77 into the bore of locking valve 73, resulting in unseating ofthe adjacent spring pressed ball valve and release of power fluid through line 79 to orbitrol 72. At the same time the opposite ball valve in locking valve 73 is unseated by the pin of the plunger in the bore of the locking valve to thereby permit fluid to return from orbitrol 72 through line 80, locking valve 73 and line 81 to sump 75.

ln the event ofa failure of pressure in conduit 77, the plunger of locking valve 73 moves toward the central position as shown for plunger 91 in locking valve 74 permitting both spring pressed ball valves to close. and in effect disconnects orbitrol control unit 72 from the fluid pressure source 71. Manual steering by turning of wheel 93 is thereafter effected by actuation of the orbitrol in transferring pressure fluid between conduits 82 and 83 depending upon the direction of steering wheel rotation to effect a corresponding expansion and contraction of the port and starboard cylinders 18 and steering of the boat.

The locking valve 73 serves to retain the fluid in the orbitrol control system during the manual steering just described. Additionally when the fluid pressure source 71 is operative to supply fluid under pressure through locking valve 73 and conduit 79 to the orbitrol unit 72, fluid in the control system is continually replenished to compensate for leakage of fluid from the orbitrol unit back through passage and locking valve 73.

The pressure source 71 functioning through locking valve 73 tends to constitute to steering assist mechanism whereby steering wheel 93 is easy to operate. When pressure source 71 fails. operation of the steering wheel 93 becomes more difficult but is still feasiblev Referring to FIG. 8, the trim cylinder 12 is connected at its opposite ends by conduits 94 and 95, respectively to a source 96 of power fluid.

The fluid source 96 basically comprises a reversible motor driven gear pump 97 connected to receive hydraulic fluid from a reservoir 98 in either direction of operation.

For this purpose. passage 99 supplies fluid from reservoir 98 to pump 97 when the latter operates in a direction to effect outward and upward tilt of tilt housing 8, and passage 100 supplies fluid from reservoir 98 to pump 97 when the latter operates in a direction to effect downward and inward movement of tilt housing 8.

Each passage 99 and 100 has a floating ball check valve 101 therein to prevent fluid being discharged by pump 97 from flowing directly to the reservoir 98.

The pump 97 discharges fluid received from passage 99 into the up-line conduit 94 through a check valve 102 which maintains the pressure in the up-line conduit 94 when the pump 97 is stopped The pump 97 discharges fluid received from passage 100 into the down-line conduit 95 through a check valve 103 which maintains the pressure in the downline conduit 95 when the pump 97 is stopped.

When the up-line conduit 94 is pressurized by pump 97, the cylinder chamber 104 on the front side of the piston 15 becomes pressurized and the piston is forced outwardly of the cylinder to extend the latter and effect an upward tilting of the tilt housing 8 and drive assembly 2 on tilt axis 9. At the same time, fluid in cylinder chamber 105 on the back side of piston 15 is forced out through down line conduit 95 and passage 106 to reservoir 98.

The passage 106 has a variable resistant spring biased ball check valve 107 which allows fluid to pass at low velocities but checks the flow when a predetermined high velocity is reached.

Between valve 107 and reservoir 98, the passage 106 has an adjustable pressure relief check valve 108 for the purpose of maintaining a predetermined pressure on the back side of piston 15 to prevent trail-out and also provide the necessary transfer of thrust of the pro peller during reverse rotation of it.

When the downdine conduit 95 is pressurized by pump 97. the cylinder chamber 105 becomes pressurized to force the piston 15 inwardly ol the cylinder 12 thereby contracting the latter and effecting a downward tilt movement of housing 8 and drive assembly 2 on tilt axis 9. At the same time, fluid in cylinder chattiher 104 is forced out through up-linc conduit 94 and check valve 102 to passage leading to the inlet of pump 97. For this purpose check valve 102 is forced open by a pilot plunger [09 actuated by fluid pressure from the discharge side of pump 97.

An adjustable pressure relief valve 110 by-passes check valve 101 in passage 100 to relieve the excess pressure in cylinder chamber 104 at the end of the piston stroke when the drive assembly 2 has been raised to its highest tilt position and before pump 97 is stopped.

An adjustable pressure relief valve 111 connects upline conduit 94 to reservoir 98 and is normally set to open at a high pressure to protect the system from possible external overload forces.

The piston [S has a plurality of circumferentially spaced spring biased check valves 112 to prevent flow of fluid through the piston from chamber 104 to chamber 105, and to allow a restricted flow of fluid through the piston from chamber 105 to chamber 104 in the event chamber [05 becomes excessively pressurized by external forces sufficient to substantially close check valve 107.

Lateral forces on the drive assembly 2 are transmitted to the tilt housing 8 and supported by the thrust washers H3 encircling the base of the trunnions 10 between the tilt housing 8 and the transom plate 3 at the upper end of the housing as shown in FIG. 3, and by the vertical side surfaces 114 of connections at the lower end of housing 8 bearing against complementary rearwardly extending vertical guide surfaces 115 of the transom plate 3 as shown in FIG. 6.

As shown also in FIG. 6, the upper gear housing 32 is constructed to provide stops 116 disposed on either side to be engaged by the corresponding steering abutment rings 69 to limit the angular displacement of dirigible unit 33 for steering purposes.

In the construction illustrated. the steering or dirigible unit 33 is entirely suspended from upper gear housing 32 by means of the vertical drive shaft 36. Thus. the same bearings generally serve to support the shaft 36 and also the dirigible unit 33. The assembly of the drive assembly 2 is thereby made lighter and more compact, and the assembly and disassembly thereofis facilitated.

The low mounting of the stern drive on the boat transom and the relative short angular tilt movement limited by the dimensions of the trim cylinder 12 and its location relative to tilt axis 9 generally prevents the propeller 62 from reaching an inoperative position upon upward tilting of the unit.

Various modes ofcarrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

We claim:

l. A marine drive for boats having a dirigible unit and means for mounting the dirigible unit on a boat; by draulic steering means for said dirigible unit and com prising a source of fluid pressure having interconnected sump and pump means. a double acting power cylinder means carried by said mounting means and connected to said dirigible unit to steer the same upon a generally vertical axis, an orbitrol helm connected to the sump and pump means of said power source to selectively transmit pressure fluid from said source to the power cylinder means and to return spent fluid from said cylinder means to the sump. and a locking valve having check valve in the connections between said source and said orbitrol helm to isolate the pressure fluid in said helm and power cylinder means from said sump and pump means in the event of failure of said source.

2. The construction of claim I and a locking valve disposed between said helm and said power cylinder means to confine the power fluid in the power cylinder means for the duration of any angular setting for the di rigible unit.

3. [n a stern drive for boats having a dirigible propulsion unit and means for mounting said unit upon the transom of a boat; hydraulic steering means for said propulsion unit and comprising a power cylinder disposed on each side of said propulsion unit and carried by said mounting means, abutment means on said dirigible unit interengaging with said power cylinders. a source of fluid pressure having interconnected sump and pump means, an orbitrol helm connected to the sump and pump means of said power source to selectively transmit pressure fluid from said source to the power cylinders and to return spent fluid from said cylinders to the sump, and a locking valve having check valves in the connections between said source and the orbitrol helm to completely isolate the pressure fluid in said helm and power cylinders from said sump and pump means in the event of failure of said source.

4. In a stern drive for boats, a mounting assembly adapted to be secured to the transom of a boat and to provide a drive connection from an inboard engine; a drive assembly removably secured to said mounting assembly and comprising a substantially vertical drive shaft having means connecting the same to said drive connection; an underwater dirigible unit suspended solely by said drive shaft and having a propeller at the lower end thereof operatively connected to the drive shaft; hydraulic steering means for said dirigible unit and comprising a power cylinder disposed on each side of the drive assembly between said mounting assembly and said dirigible unit to control the steering of said unit upon the axis of the drive shaft, a source of fluid pressure having interconnected sump and pump means, an orbitrol helm connected to the sump and pump means of said power source to selectively transmit pres sure fluid from said source to said power cylinders and to return spend fluid from said cylinders to the sump, and a locking valve having check valves in the connections between said source and the orbitrol helm to confine the pressure fluid in said helm and power cylinders completely isolated from the sump and pump means of said power source in the event of failure of said source. 

1. A marine drive for boats having a dirigible unit and means for mounting the dirigible unit on a boat; hydraulic steering means for said dirigible unit and comprising a source of fluid pressure having interconnected sump and pump means, a double acting power cylinder means carried by said mounting means and connected to said dirigible unit to steer the same upon a generally vertical axis, an orbitrol helm connected to the sump and pump means of said power source to selectively transmit pressure fluid from said source to the power cylinder means and to return spent fluid from said cylinder means to the sump, and a locking valve having check valve in the connections between said source and said orbitrol helm to isolate the pressure fluid in said helm and power cylinder means from said sump and pump means in the event of failure of said source.
 2. The construction of claim 1 and a locking valve disposed between said helm and said power cylinder means to confine the power fluid in the power cylinder means for the duration of any angular setting for the dirigible unit.
 3. In a stern drive for boats having a dirigible propulsion unit and means for mounting said unit upon the transom of a boat; hydraulic steering means for said propulsion unit and comprising a power cylinder disposed on each side of said propulsion unit and carried by said mounting means, abutment means on said dirigible unit interengaging with said power cylinders, a source of fluid pressure having interconnected sump and pump means, an orbitrol helm connected to the sump and pump means of said power source to selectively transmit pressure fluid from said source to the power cylinders and to return spent fluid from said cylinders to the sump, and a locking valve having check valves in the connections between said source and the orbitrol helm to completely isolate the pressure fluid in said helm and power cylinders from said sump and pump means in the event of failure of said source.
 4. In a stern drive for boats, a mounting assembly adapted to be secured to the transom of a boat and to provide a drive connection from an inboard engine; a drive assembly removably secured to said mounting assembly and comprising a substantially vertical drive shaft having means connecting the same to said drive connection; an underwater dirigible unit suspended solely by said drive shaft and having a propeller at the lower end thereof operatively connected to the drive shaft; hydraulic steering means for said dirigible unit and comprising a power cylinder disposed on each side of the drive assembly between said mounting assembly and said dirigible unit to control the steering of said unit upon the axis of the drive shaft, a source of fluid pressure having interconnected sump and pump means, an orbitrol helm connected to the sump and pump means of said power source to selectively transmit pressure fluid from said source to said power cylinders and to return spend fluid from said cylinders to the sump, and a locking valve having check valves in the connections between said source and the orbitrol helm to confine the pressure fluid in said helm and power cylinders completely isolated from the sump and pump means of said power source in the event of failure of said source. 